Conventionally, gallium nitride (GaN)-based Group III-V compound semiconductors of a zinc-blende crystal type or a Wurtzite crystal type are employed for fabricating semiconductor devices, such as a light-emitting device emitting visible light of short wavelength (see, for example, JP-A HEI 2-288388). Stacked structures for fabricating gallium-nitride-based semiconductor devices are produced by use of a single-crystal substrate composed of a high-heat-resistance aluminum oxide, such as sapphire (α-Al2O3 single crystal) or a garnet single-crystal (see, for example, JP-A HEI 7-288231).
The above single-crystal substrate composed of aluminum oxide (e.g., sapphire) has lattice constants considerably different from those of a gallium nitride (GaN)-based semiconductor material. Thus, the stacked structure for use in fabrication of gallium-nitride-based semiconductor devices is generally formed via a buffer layer on a single-crystal substrate. The buffer layer for mitigating mismatch in terms of lattice constants is generally called a “low-temperature-deposited buffer layer” since the layer has been conventionally formed at relatively low temperature (see, for example, Book edited by Isamu AKASAKI, “Group III-V Compound Semiconductors,” published by Baifukan Co., Ltd., 1st edition, Chapter 13, (May 20, 1995)).
The low-temperature-deposited buffer layer is formed from, for example, gallium nitride (see, for example, JP-A HEI 8-255926). In order to mitigate lattice constant mismatch with respect to a crystalline substrate, the low-temperature-deposited buffer layer is preferably formed of an amorphous material or a polycrystalline material in an as-grown state (see, for example, JP-A HEI 8-255926). Meanwhile, there has been disclosed another technique for forming a low-temperature-deposited buffer layer of a layer having a junction area with respect to a crystalline substrate formed in an as-grown state into a single-crystal layer (see, for example, JP-A HEI 10-321905).
However, the low-temperature-deposited buffer layer formed on the surface of a sapphire substrate has a drawback in that the orientation of the single-crystal layer included in the buffer layer is not sufficiently unified on the surface of the sapphire substrate, thereby failing to attain consistent formation, on the substrate, of a GaN-based Group III nitride semiconductor layer having a well-unified orientation and excellent single-crystal characteristics.
Thus, an object of the present invention is to provide a crystal structural feature of a single-crystal layer included in a low-temperature-deposited buffer layer so as to form, on a low-temperature-deposited buffer layer of AlXGaYN (0<X, Y<1, X+Y=1) having a single-crystal layer included in the vicinity of a junction interface area thereof in contact with a substrate, a gallium-nitride-based semiconductor layer having excellent single-crystal characteristics that is suitably grown. Another object of the invention is to provide a stacked structure having a GaN-based nitride semiconductor layer of excellent single-crystal characteristics that is formed via a low-temperature-deposited buffer layer including a single-crystal layer having the above crystal feature. Yet another object of the invention is to provide a compound semiconductor device exhibiting excellent characteristics by virtue of employment of the stacked structure.